1. Field of the Invention
The present invention relates to recording apparatus and method, and more particularly to temperature control and accompanied process, particularly temperature overrise protection, in the recording apparatus.
2. Related Background Art
A recording apparatus used in a printer, a copying machine and a facsimile machine is constructed to record an image comprising a dot pattern on a recording medium such as a paper or a plastic thin sheet in accordance with recording information. Such recording apparatus may be classified by recording systems thereof into ink jet system, wire dot system, thermal transfer system and laser beam system. Of those, the ink jet system recording apparatus discharges ink droplets (recording liquid) from discharge ports (outlets or orifices) provided in a head and deposits them to a recording medium to record an image and it has been widely used because it satisfactorily meets general requirements of high speed recording, high resolution recording, high grade recording and low noise recording.
As a general construction to meet the above requirements, a higher head drive frequency and a larger number of recording elements are used. In such a case, an energy applied to the recording head remarkably increases.
Particularly, in the ink jet system in which air bubbles are generated in ink by using thermal energy to discharge ink droplets, this tendency is remarkable. For example, in the recording apparatus, a member for mounting a recording head, an ink tank, and a member for supplying ink serve to emit heat by the energy application, and when the drive frequency is doubled while a volume and a surface area of those members are kept fixed, a relatively double energy would be applied. When the number of discharge ports is doubled, a double energy would be applied, similarly. In actuality, when the number of recording elements or discharge ports is increased, a volume near the discharge ports increases but a volume of other parts and a surface area thereof do not significantly increase. Thus, in the above case, approximately four times energy would be applied to the substantially constant volume and surface area.
In this case, for the ink jet system using the thermal energy, several tens percent of the applied energy is emitted out of the recording head by a kinetic energy to discharge the ink and heat generation by the discharged ink. Thus, approximately two times temperature rise is generated in the recording head by the application of the four times energy.
However, the temperature rise in such a recording head raises the following two problems.
A first problem is due to the fact that the temperature of the recording head rises high by the approximately two times temperature rise.
For example, when recording is made at a relatively high recording duty in an environment temperature of 30.degree. C., a temperature in the apparatus rises approximately 10.degree. C. by the temperature rise of a power supply, a motor and a driver in the recording apparatus. In this case, if a recording head with a relatively low drive frequency and a relatively small number of discharge ports is used, the temperature rise will be approximately 25.degree. C. even for full painting or 100% duty recording, but when the drive frequency is doubled and the number of discharge ports of the recording head is doubled, the temperature rise will be double, that is, approximately 50.degree. C. By summing the environment temperature and the temperature rises, the temperature of the recording head is approximately 65.degree. C. for the low drive frequency and the small number of discharge ports while it is approximately 90.degree. C. for the high drive frequency and the large number of discharge ports.
When the temperature of the recording head reaches approximately 90.degree. C., the failure of discharge is apt to occur. Further, in an apparatus in which the recording head is exchangeable or it may be touched by a user, it is necessary to pay attention to prevent the user from touching the recording head while the recording head is at a high temperature.
A second problem relates to a break mode of the recording head.
As explained above, the recording head temperature may reaches 90.degree. C. depending on the recording status. In this case, even if four times energy is applied to the recording head, the temperature rise thereof is approximately two times because several tens percent of energy is ejected out of the recording head as the thermal and kinetic energies when the ink is discharged. However, although the operation is that described above when the ink is normally discharged, when the ink is not supplied to the recording head when an ink tank is empty and no ink to be discharged is present or when air bubbles stay in an ink supply path to block the supply of the ink, a so-called empty heat state in which the recording head is driven without ink takes place.
In this case, since four times energy is supplied with no ink discharged in the above example, the energy for the discharge of the ink causes the abrupt rise of the temperature of the recording head so that the temperature of the recording head reaches one hundred and several tens .degree.C. As a result, plastic parts of the recording head exceed a thermal deformation temperature and they may be deformed, adhered portions may be torn off by the abrupt thermal expansion or the ink remaining near the heater is burnt making the heater inoperable.
The break mode which is inherent to the ink jet system is different from a break mode in the conventional thermal transfer system or wire dot system in which the temperature gently rises by the continuous recording to cause the break due to the temperature overrise determined by a heat capacity of the recording head unit. The existence of such a break mode makes difficult the solution by various countermeasures for the conventional break mode.